Work is continuing on the structures and modes of action of various ADP-ribosylating exotoxins. Diphtheria toxin will be studied in detail, with the goal of elucidating its 3-dimensional structure and the mechanism by which the A chain crosses membranes and enters the cytoplasm of sensitive cells. We shall attempt to identify an endogenous, tightly bound nucleotide which is associated with one form of the toxin, and to use the nucleotides as a probe of toxin structure and function. We shall attempt to identify active site peptides from diphtheria toxin and other related toxins. Studies on the nucleotide sequence of the structural gene for diphtheria toxin will continue.